1. Field of the Invention
The present invention relates to a motor controller and more specifically to a motor controller configured to control a switched reluctance motor.
2. Description of the Related Art
Switched reluctance motors have been mentioned recently because of their advantage of low manufacturing costs, and simple rugged structure. A switched reluctance motor (hereinafter, referred to as “SRM”) has no rotor winding and no permanent magnet, and has excellent robustness at high speeds and a wide speed range. In an SRM, torque is generated by applying current to stator windings while inductance is changing. If current is applied when the inductance is increasing, positive torque is created, while if current is applied when inductance is decreasing, negative torque is created.
To control the SRM efficiently, individual algorithms are usually used in high, middle and low speeds. For example, in a low speed, a chopping mode control is used, where a discrete constant current is applied to each phase. In a high speed, another control which is called angle control is used. The switching pattern of this control is called “single-pulse mode” where a single pulse is applied to each cycle of inductance. In a middle speed, a mixed control of low and high speeds is used.
To achieve the required current pattern for an SRM, various special inverters are proposed. For example, Zeljko et al. (“A Novel Power Inverter for Switched Reluctance Motor Drives”, Elec. Energ. Vol. 18, No. 3, December 2005, pp. 453-465) discloses a topology where four inverter legs are provided for a three-phase SRM supply. The fourth leg is utilized to connect a star point of a three-phase winding. Because of this topology, each phase can be controlled independently.
On the other hand, WO2011/65406 discloses a direct torque control (hereinafter, referred to as “DTC”) system for an interior permanent magnet motor. In the DTC system, one of switching patterns for the inverter is selected in accordance with a torque, a reference torque, a flux, a reference flux, a phase angle and so on. The reference flux is obtained by referring to a flux trajectory which shows a relationship between flux magnitude and phase angle. The DTC system, shown in WO2011/65406, varies the form of the flux trajectory between a circle and a polygon in accordance with torque, reference torque, motor speed and crossover speed.
Conventional inverters for SRM have special topologies; therefore, the inverters should have discrete components. Using discrete components in an inverter instead of a single module increases cost, weight, size, assembling time and complexity, and reduces the inverter reliability and its ruggedness. Besides, the inverters cannot be used for any other type of motors.
Furthermore, inverters for SRM need individual algorithms for high, middle and low speeds, which increase the complexity of the system and setting time.
In a motor controller for SRM disclosed in WO2013/105506, one of switching patterns is selected based on a comparison result of reference torque and estimated torque and a comparison result of reference flux and estimated flux and a flux phase angle and inputting it to an inverter. The reference flux is obtained from a flux trajectory and a rotor angle with changing the flux trajectory in accordance with the reference torque and rotating speed, the flux trajectory indicating a relationship between rotor angle and reference flux.